This is a continuation of Application Ser. No. 437,308, filed Oct. 28, 1982, now abandoned.
The present invention relates to a construction of an industrial robot and, more particularly, to an arm driving mechanism and wrist mechanism for an industrial robot.
Recently, there has been progress in automatically assembly works by industrial robots in production processes and, for example, FIG. 1 provides a typical conventional industrial robot.
As shown in FIG. 1, an industrial robot is provided with two articulations, a first arm 1 and a second arm 2, with the first arm 1 being driven directly by a D.C. servo motor 3 provided on the base end thereof while the second arm 2, pivotally connected to the free end of the first arm 1, being directly driven by a D.C. servo motor 4 provided on the base end thereof. A pneumatic cylinder 5, secured to the free end of the second arm 2 has a piston rod to the end of which attached is a wrist 6 so that the wrist 6 is moved up and down as the piston rod of the pneumatic cylinder is extended and retracted. A pulse motor 7, fixed to the base 8, transmits the torque through timing belts 9, 9', 9" to the wrist thereby causing a twisting action of the wrist.
The robot shown in FIG. 1 has two degrees of freedom attained by the first and the second rams 1, 2 within a horizontal plane, one degree of freedom in the vertical direction and one degree of freedom attained by the twisting of the wrist 6. One disadvantage of the industrial robot of the above described construction resides in the fact that, since the vertical movement of the wrist 6 is caused by a pneumatic cylinder, it is not possible to locate and fix the wrist 6 at the desired position in the vertical direction. It is true that the wrist 6 can be correctly stopped at its upper and lower stroke ends by providing with suitable stoppers but any change of the stopping positions necessitates a change of the stoppers. Thus, it is not easy to adapt this industrial robot to various operation patterns.
Another disadvantage resides in the fact that there is a large rotational inertia of the rotational members thereby making it difficult to attain high speed operation and to improve the precision of the stopping positions unless a drive motor of a drive capacity is used. One of the factors causing the large rotational inertia is the mass of the D.C. servo motor 4 on the free end of the first arm 1. Namely, it is necessary to move and swing the D.C. servo motor 4 when the first arm is swung.
In order to eliminate this problem, it has been proposed to fix the motor for swinging the second arm 2 to a stationary member and connecting the same to the second arm through a connecting rod and a crank. This transmission mechanism incorporating a crank and a connecting rod requires that the rotation angle of the second arm is considerably smaller than 180.degree.. Namely, if the rotational angle is increased nearly to 180.degree., a dead point is formed at the stroke end to disable the transmission mechanism from transmitting the power.
More particularly, as shown in FIG. 2, a conventional driving mechanism includes a crank 11 and a link 12 through which the power is transmitted from a motor 10 to the arm 13 to drive the latter. In this driving mechanism, the force F' for driving the arm 13 is decreased as the arm is rotated, i.e. as the range of movement of the arm is increased, as will be seen from FIGS. 3a and 3b.
Representing the force from the motor by F and the angle of rotation of the arm by .theta., the force F' for driving the arm is expressed as follows: EQU F'=F cos.theta..
When the rotation angle .theta. is 90.degree., the driving force F' is nullified, i.e. becomes zero, so that the arm cannot be driven. Thus, in the arm driving mechanism incorporating a single link, the pressure angle of the arm driving force is increased as the range of rotation of the arm is increased, so that the load imposed on the motor is increased when the arm is rotated to a position of large rotation angle. Thus, the arm driving mechanism incorporating a single link inherently involves problems concerning the load imposed on the motor and the range of rotation of the arm.
Accordingly, it is an object of the present invention to provide an improved industrial robot which increases the range of rotation of the arm and reduces the load imposed on the motor, to thereby overcome the above-described disadvantages of the prior art.
It is another object of the invention to provide an improved industrial robot which enables a realization of a high precision stopping position of the wrist in the vertical direction while reducing the rotational inertia of the swingable portion to permit a small capacity motor to drive the arm at a high speed.
It is still another object of the invention to provide an industrial robot provided with a wrist device having three degrees of freedom in which the mechanical transmission between the wrist device and the driving device attached to an arm of the like member is simplified to realize a compact and small-sized construction of the industrial robot.
To these ends, according to the invention, the output power of the motor is transmitted to the arm through a double link mechanism having a phase difference of motion. More particularly, the double link mechanism includes a pair of cranks and a pair of links. The combination of the cranks and links transmits the power of the motor with a certain phase difference and, according to this arrangement, it is possible to increase the range of rotation of the arm while reducing the load imposed on the motor, to thereby overcome the problem of the prior art.
The links of the double link mechanism are arranged to avoid interference with the arms and the shafts of the cranks to further increase the range of rotation of the robot arm.
Furthermore, two links are arranged such that the distance between two links is minimized when the first and the second arms are stretched to make their longitudinal axes extend along a line. The rotation of the arm of the robot is commenced or ended in this state. This arrangement permits further increase of the range of rotation of the robot arm.
According to another aspect of the invention, the industrial robot has a motor mounted on a stationary member, winding power transmitting means through which the output power is transmitted to a second arm, and mechanicl conversion means provided on the end of the second shaft and adapted for converting a rotational motion into a linear motion, whereby a tool holding shaft is reciprocatingly moved up and down by the rotational output power derived from the motor. In this arrangement, a drive motor for rotating the second arm is mounted on a stationary member so that the rotational inertia of the swingable or rotational members is reduced. The above mentioned double link mechanism is used to transmit the output power of this motor to the second arm, to thereby ensure smooth rotation of the arm.
According to still another aspect of the invention, the wrist device of the industrial robot is provided with a mechanical power transmitting mechanism for drivingly connecting a transmission shaft concentric with the axis of rotation of the rotary housing to a drive shaft which is concentric with or parallel to the transmission shaft and for permit the transmission of power between the shafts regardless of the posture of an oscillation housing.